1. Field of the Invention
The field of the invention is an intake system of an internal combustion engine, which has first and second intake valve openings provided in an engine body so as to be exposed to a combustion chamber, first and second intake valves disposed in the engine body and capable of opening and closing the first and second intake valve openings, respectively, and a valve operating system connected to said first and second intake valves and capable of changing operational modes of the valves in compliance with the operational condition of the engine.
2. Description of the Prior Art
Intake systems of the above mentioned type are known, for example, from Japanese Patent Application Laid-open No. 252832/86 and the like.
In such a conventional system, when an engine is operating at a low speed, both the intake valves are operated in a mode corresponding to the low speed operation of the engine and the area of intake passage is reduced by a swirl control valve to provide a swirl effect for reducing the fuel consumption. On the other hand, at the time of a high speed operation of the engine, both the intake valves are operated in a mode corresponding to the high speed operation of the engine and the area of the intake passage is increased by the swirl control valve so as to improve the charging efficiency and thereby enhance the output performance of the engine.
In such an intake system, it is arranged that an intake port leading to one of the intake valve openings is closed by the swirl control valve in the low speed operation of the engine thereby to admit an air-fuel mixture into the combustion chamber only through the other intake valve opening so as to provide the swirl effect. In this arrangement, however, the intake valve which is capable of opening and closing the one of the intake valve openings continues its opening and closing actions in a mode corresponding to the low speed operation of the engine and opening and closing of the one intake valve opening leading to the combustion chamber may undesirably disorder the swirl in the combustion chamber. Further, in that case, since the one of the intake valves which has substantially nothing to do with controlling intake of the air-fuel mixture to the combustion chamber during the low speed operation is still operated for opening and closing actions, additional driving force is required for obtaining such actions. Furthermore, in the conventional system, it is necessary to dispose the swirl control valve in the intake port, and provision of such a valve increases the intake resistance at the time of high speed operation of the engine, and this could be a factor of disturbing the improvement in the engine output performance.